2019
Publication Year
2021-04-21T10:05:35Z
Acceptance in OA@INAF
Organic molecules in the protoplanetary disk of DG Tauri revealed by ALMA
Title
PODIO, LINDA; BACCIOTTI, Francesca; FEDELE , DAVIDE; Favre, C.;
CODELLA, CLAUDIO; et al.
Authors
10.1051/0004-6361/201834475
DOI
http://hdl.handle.net/20.500.12386/30818
Handle
ASTRONOMY & ASTROPHYSICS
Journal
623
Number
https://doi.org/10.1051/0004-6361/201834475 c ESO 2019
Astronomy
&
Astrophysics
L
ETTER TO THEE
DITOROrganic molecules in the protoplanetary disk of DG Tauri revealed
by ALMA
L. Podio
1, F. Bacciotti
1, D. Fedele
1, C. Favre
1, C. Codella
1,2, K. L. J. Rygl
3, I. Kamp
4, G. Guidi
5, E. Bianchi
2,
C. Ceccarelli
2, D. Coffey
6,7, A. Garufi
1, and L. Testi
8,11 INAF – Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy e-mail: lpodio@arcetri.astro.it
2 Univ. Grenoble Alpes, IPAG, 38000 Grenoble, France
3 INAF–Istituto di Radioastronomia & Italian ALMA Regional Centre, via P. Gobetti 101, 40129 Bologna, Italy 4 Kapteyn Astronomical Institute, University of Groningen, Landleven 12, 9747 AD Groningen, The Netherlands 5 ETH Zurich, Institute for Particle Physics and Astrophysics, Wolfgang-Pauli-Str. 27, 8093 Zurich, Switzerland 6 School of Physics, University College Dublin, Belfield, Dublin 4, Ireland
7 School of Cosmic Physics, The Dublin Institute for Advanced Studies, Dublin 2, Ireland 8 European Southern Observatory (ESO), Karl-Schwarzschild-Str. 2, 85748 Garching, Germany
Received 19 October 2018/ Accepted 3 February 2019
ABSTRACT
Context. Planets form in protoplanetary disks and inherit their chemical compositions.
Aims. It is thus crucial to map the distribution and investigate the formation of simple organics, such as formaldehyde and methanol, in protoplanetary disks.
Methods. We analyze ALMA observations of the nearby disk-jet system around the T Tauri star DG Tau in the o − H2CO 31,2− 21,1 and CH3OH 3−2,2− 4−1,4E, 50,5− 40,4A transitions at an unprecedented resolution of ∼0.0015, i.e., ∼18 au at a distance of 121 pc.
Results. The H2CO emission originates from a rotating ring extending from ∼40 au with a peak at ∼62 au, i.e., at the edge of the 1.3 mm dust continuum. CH3OH emission is not detected down to an rms of 3 mJy beam−1in the 0.162 km s−1channel. Assuming an ortho-to-para ratio of 1.8−2.8 the ring- and disk-height-averaged H2CO column density is ∼0.3−4 × 1014cm−2, while that of CH3OH is <0.04−0.7 × 1014cm−2. In the inner 40 au no o − H
2CO emission is detected with an upper limit on its beam-averaged column density of ∼0.5−6 × 1013cm−2.
Conclusions. The H2CO ring in the disk of DG Tau is located beyond the CO iceline (RCO ∼ 30 au). This suggests that the H2CO abundance is enhanced in the outer disk due to formation on grain surfaces by the hydrogenation of CO ice. The emission peak at the edge of the mm dust continuum may be due to enhanced desorption of H2CO in the gas phase caused by increased UV penetration and/or temperature inversion. The CH3OH/H2CO abundance ratio is <1, in agreement with disk chemistry models. The inner edge of the H2CO ring coincides with the radius where the polarization of the dust continuum changes orientation, hinting at a tight link between the H2CO chemistry and the dust properties in the outer disk and at the possible presence of substructures in the dust distribution.
Key words. protoplanetary disks – astrochemistry – ISM: molecules – stars: individual: DG Tau
1. Introduction
A key open question in astrochemistry is how chemical com-plexity increases during the formation process of Sun-like stars from prestellar cores to protoplanetary disks and ultimately to planets (Caselli & Ceccarelli 2012). Is the chemical composition of planets inherited from the prestellar and protostellar stages? Or does it reflect chemical processes occurring in the disk? Are organics efficiently formed in disks and by what mechanism(s)? Organic and prebiotic molecules form either through gas-phase reactions (e.g.,Millar et al. 1991;Balucani et al. 2015) or on the icy surface of dust grains (e.g., Tielens & Hagen 1982;
Garrod et al. 2008). Understanding the efficiency and occurrence of these mechanisms requires a comparison of observations and predictions from astrochemical models. Detailed models of the disk chemistry including gas-phase reactions, molecule freeze-out, dust surface chemistry, and both thermal and non-thermal desorption mechanisms have been developed to predict the ice and gas abundances of complex organic molecules (e.g.,
Aikawa & Herbst 1999; Willacy & Woods 2009; Walsh et al. 2014;Loomis et al. 2015). However, only a few simple organics have been detected in disks so far (Qi et al. 2013; Öberg et al. 2015a; Walsh et al. 2016; Favre et al. 2018). Among these organics, formaldehyde (H2CO) and methanol (CH3OH) are
essential to the investigation of the formation of organics. While H2CO can form both in the gas phase and on grains, CH3OH
forms exclusively on grains (e.g., Watanabe & Kouchi 2002). Therefore, observations of its abundance and distribution in disks is crucial to constraining the mechanism(s) that form these simple organics, which are the building blocks in the formation of complex organic and prebiotic molecules.
Observationally, H2CO has been detected in a number of
pro-toplanetary disks through single-dish and low angular resolution surveys (e.g., Öberg et al. 2010, 2011; Guilloteau et al. 2013;
Qi et al. 2013), but only three of them were imaged at an angular resolution of ∼0.500, namely DM Tau (Loomis et al. 2015), TW Hya (Öberg et al. 2017), and HD 163296 (Carney et al. 2017). Therefore, the distribution and formation mechanisms (whether
A&A 623, L6 (2019) Table 1. Properties of the observed lines.
Line ν0 Eup Si jµ2 HPBW (PA) rms Fint NX
(MHz) (K) (D2) (mJy beam−1) (mJy km s−1) (1014cm−2)
o-H2CO 31,2− 21,1 225 697.775 33 43.5 0.0017 × 0.0013 (−20◦) 1.7 292 0.2−2.7
CH3OH 3−2,2− 4−1,4E 230 027.060 40 0.7 0.0016 × 0.0013 (−21◦) 3 <1.6 <0.4−10
CH3OH 50,5− 40,4A 241 791.431 35 4 0.0016 × 0.0012 (−21◦) 3 <0.7 <0.04−0.7
in the gas phase or on grains) of H2CO in disks is still unclear.
Because of its low volatility and large partition function, CH3OH
is even more difficult to observe. To date, it has been detected only in the disks of TW Hya (Walsh et al. 2016) and of the young outbursting star V883 Ori (van ’t Hoff et al. 2018).
The nearby T Tauri star DG Tau (d = 121 ± 2 pc;
Gaia Collaboration 2016,2018) is an ideal target to investigate the origin of simple organics in protoplanetary disks. DG Tau is surrounded by a compact and massive dusty disk imaged with CARMA (Isella et al. 2010) and ALMA in polarimet-ric mode (Bacciotti et al. 2018), and is associated with bright molecular emission detected with the IRAM 30 m telescope and Herschel(Guilloteau et al. 2013;Podio et al. 2012;Fedele et al. 2013). However, the origin of the detected molecular emission is unclear because DG Tau is also associated with a residual enve-lope and a jet (Eislöffel & Mundt 1998).Guilloteau et al.(2013) suggest that the single-peaked profile of SO and H2CO is due
to envelope emission, while the fundamental H2O lines have
double-peaked profiles and fluxes in agreement with disk model predictions (Podio et al. 2013). Interferometric maps of CO and its isotopologues show that the envelope dominate the molecu-lar emission on molecu-large scales (Schuster et al. 1993;Kitamura et al. 1996), while disk emission is detected on scales <200(Testi et al. 2002; Güdel et al. 2018). Given the complexity of the circum-stellar environment, spatially resolved maps are crucial to reveal the origin of the molecular emission.
In this letter we present ALMA Cycle 4 observations of H2CO and CH3OH in the disk of DG Tau at an unprecedented
resolution of ∼0.0015, i.e., ∼18 au, and we discuss the possible
formation mechanism of these simple organics. 2. Observations
ALMA observations of DG Tau were performed during Cycle 4 in August 2017 with baselines ranging from 17 m to 3.7 km. The bandpass was calibrated with the quasar J0510+1800, and phase calibration was performed every ∼8 min using quasar J0438+3004. The correlator setup consists of 12 high-resolution (0.122 MHz) spectral windows (SPWs) covering several molec-ular transitions, among which o-H2CO 31,2− 21,1, and CH3OH
3−2,2 − 4−1,4 E and 50,5 − 40,4 A (frequency, ν0; upper level
energies, Eup; and line strengths, Si jµ2 are listed in Table 1).
Data reduction was carried out following standard procedures using the ALMA pipeline in CASA 4.7.2. Self-calibration was performed on the source continuum emission, by combining a selection of line-free channels, and applying the phase solu-tions to the continuum-subtracted SPWs. Continuum images and spectral cubes were produced with tclean applying a manually selected mask and Briggs parameter of 0.5. The flux calibra-tion was performed using quasars J0238+1636 and J0510+1800, obtaining an accuracy of ∼10%. The clean beam FWHMs and r.m.s (per 0.162 km s−1 channel) of the resulting line cubes are listed in Table 1. The continuum image has an rms of 0.3 mJy beam−1.
3. Ring of formaldehyde in the disk of DG Tau The map of the continuum emission at 225.7 GHz (1.3 mm) (Fig. 1) shows a smooth distribution, with integrated flux of 297 mJy and peak intensity of 33 mJy beam−1 at RA(J2000)= 04h27m04s.70, Dec(J2000) = +26◦06015.0072. By applying a 2D
Gaussian fit, we determine the disk major and minor axes decon-volved from the beam (0.0043 ± 0.0002 and 0.0038 ± 0.0002), hence the
disk inclination i= 41◦± 2◦and position angle PA= 128◦± 16◦.
These values are in good agreement with the estimates obtained at 0.87 mm and 1.3 mm (Bacciotti et al. 2018;Isella et al. 2010) and with the jet inclination and PA (the jet is ejected perpen-dicular to the disk at ijet = 38◦ ± 2◦, PAjet = 226◦ ± 10◦,
Eislöffel & Mundt 1998).
The moment 0 and 1 maps of the H2CO 31,2− 21,1
emis-sion obtained with CASA by integrating over velocities from 3.24 to 9.24 km s−1, and masking all pixels where no emission
is detected above the 3σ level are shown in Fig. 1, while the moment 8 map obtained without applying a threshold is shown in Fig.A.2. Then radial intensity profiles of the H2CO 31,2− 21,1
line and of the 1.3 mm continuum are obtained by azimuthally averaging the intensity maps after deprojecting for i = 41◦and
PA= 128◦(see Fig.2). The H2CO 31,2−21,1moment maps show
an H2CO ring rotating around the central star which extends
from ∼40 au to beyond the edge of the 1.3 mm dust emission
(R1.3 mm = 66 au at 3σ) (see also the channel maps in Fig.A.1).
The intensity profile in Fig.2shows that the peak of the H2CO
emission is at ∼62 au (∼6.5 mJy beam−1km s−1). The 1D spec-trum integrated over a 0.0033−0.0075 ring area is centered at the
sys-temic velocity Vsys = +6.24 km s−1 and shows a double-peaked
profile (see Fig. A.3). Assuming Keplerian rotation around a 0.7 M star as in Podio et al.(2013)1 and i = 41◦, the H2CO
peaks at VLSR− Vsys= ±1.95 km s−1 indicate an emitting radius
of ∼70 au, in agreement with the position of the H2CO ring.
The H2CO line peak and integrated flux collected with the
1100 beam of the IRAM 30 m telescope (Guilloteau et al. 2013)
is about a factor of four larger than that obtained integrating the ALMA cube over an 1100 circular area. This indicates that
ALMA filters out the extended H2CO emission from the outflow
or the envelope, thus isolating the compact H2CO emission from
the disk.
4. Abundance of organic molecules
To estimate the H2CO column density, the emission is
inte-grated over a circular ring from 0.0033 to 0.0075 (40−90 au) and over the line velocity profile (between+3.24 and +9.24 km s−1).
The maps of the undetected CH3OH lines are integrated over
the same spatial region and velocity range as for the H2CO line
(i.e., the same pixels in RA, Dec, and VLSR) to recover an upper
1 The stellar mass obtained from the pre-main sequence tracks by
Siess et al.(2000) is not affected by the change in distance (140 pc in
previous studies) as the tracks are vertical in this region of the HR diagram.
Fig. 1.Moment maps of H2CO towards DG Tau. Left panel: H2CO moment 0 map. The color scale indicates the line intensity integrated over the H2CO velocity profile (VLSR = (+3.24, +9.24) km s−1) in Jy beam−1km s−1. The magenta contours indicate the 1.3 mm dust continuum emission (from 3σ with steps of 10σ). The blue and magenta ellipse in the bottom left and right corner indicate the synthesized beam for H2CO and the continuum, respectively. Right panel: H2CO moment 1 map. The color scale indicates the velocity VLSRin km s−1.
H
2CO
Cont
CO snowline polarization change H2
CO peak 0 50 100 150 R [au] 0 2 4 6 8
Flux density [mJy beam
−1 km s −1 ] 0 20 40 60 T cont [K] 0.0 0.2 0.4 0.6 0.8 1.0 1.2 R ["]
Fig. 2.Azimuthally averaged radial intensity profile of H2CO 31,2− 21,1
(in mJy beam−1km s−1, red line) and of the 1.3 mm continuum (in K, blue line). For the continuum the conversion factor from brightness tem-perature to flux density is 0.65 mJy beam−1K−1. The shaded areas indi-cate the dispersion of the intensity values around the mean along each annulus in the radial direction. The vertical black dotted lines indicate the position of the CO iceline, the change in the polarization orientation, and the H2CO emission peak.
limit on the methanol column density under the assumption that CH3OH originates from the same disk region as H2CO. The
ring-averaged o-H2CO and CH3OH column densities, No−H2CO and NCH3OH, are derived from the integrated line intensities by assuming local thermodynamic equilibrium (LTE) and opti-cally thin emission (see, e.g., Eqs. (1) and (2) in Bianchi et al. 2017), and adopting the molecular parameters and partition func-tion from the Cologne Database of Molecular Spectroscopy (CDMS,Müller et al. 2001). The assumption of LTE is clearly justified as the gas density near the midplane and in the inter-mediate disk layers where H2CO is thought to originate (see,
e.g,Walsh et al. 2014;Öberg et al. 2017, and the discussion in Sect.5) is high (from ∼108to ∼1012cm−3according to the disk
model byPodio et al. 2013), i.e., well above the critical density of the H2CO 31,2− 21,1transition (ncr ∼ 7 − 4.6 × 105cm−3 at
20−100 K, Shirley 2015). The gas temperature in these inter-mediate disk layers is between 30 and 300 K according to the same disk model. Therefore, assuming an excitation tempera-ture Tex = 30−300 K we infer an average o-H2CO column
den-sity over the ring and the disk height of ∼0.2−2.7 × 1014cm−2. Following the same procedure as for H2CO, the most stringent
constraint on the CH3OH column density is derived from the
CH3OH 241.791 GHz line (which has the greatest line strength
Si jµ2∼ 4 D2): NCH3OH< 0.04−0.7×10
14cm−2. The inferred
col-umn densities are summarized in Table1. The total H2CO
col-umn density is then obtained by assuming an ortho-to-para ratio of 1.8−2.8 (Guzmán et al. 2018): NH2CO ∼ 0.3−4 × 10
14cm−2.
Using the H2column density in the outer disk (>40 au) from the
disk model byPodio et al. (2013) (NH ∼ 0.3−1 × 1025cm−2),
we estimate the ring- and disk-height-averaged abundance of organic molecules: XH2CO ∼ 6 × 10
−12− 3 × 10−10, X
CH3OH <
0.8 × 10−12− 5 × 10−11. Finally, in the inner 40 au no o-H
2CO
is detected above 3σ. Taking into account the wider velocity range over which the emission may be spread, this provides an upper limit on the o-H2CO beam-averaged column density of
∼0.5−6 × 1013cm−2.
The inferred H2CO column density and abundance, and the
upper limits retrieved for CH3OH are in agreement with the
predictions of chemistry models of protoplanetary disks around T Tauri stars (see, e.g., Walsh et al. 2014). These models pre-dict gas-phase fractional abundances in the disk outer regions (r > 50 au) of between 10−13 and a few 10−10 for H
2CO and
CH3OH. The H2CO abundance is in agreement within a factor of
a few with that estimated in the disk of TW Hya and HD 163296 assuming Tex= 25 K (Carney et al. 2019), while it is up to two
orders of magnitude lower than that found in disks with large inner holes, for example Oph IRS48 and AB Aur where the outer disk reservoir of icy organics is directly exposed to stellar illumi-nation (van der Marel et al. 2014;Pacheco-Vázquez et al. 2016). Also, the nondetection of the targeted methanol transitions is consistent with a CH3OH/H2CO ratio <1, in agreement with the
prediction of chemical models (e.g.,Walsh et al. 2014) and with the upper limit found in the disk of HD 163296 (CH3OH/H2CO
< 0.24), while it is lower than that estimated in the disk of TW Hya (CH3OH/H2CO= 1.27) (Carney et al. 2019).
A&A 623, L6 (2019)
5. H2CO formation
Disk chemistry models byWillacy & Woods(2009),Walsh et al.
(2014) andLoomis et al.(2015) show that H2CO can form either
in the gas phase, primarily through the reaction CH3+ O; or on
dust grains via hydrogenation of CO locked in their icy man-tles (CO+ H –> HCO + H -> H2CO). The gas-phase CH3+ O
formation route is efficient in the warm inner region and upper layers of the disk where atomic oxygen is produced by photodis-sociation of gas-phase CO. On the contrary, formation on grains occurs in the cold outer disk beyond the CO iceline where CO is condensed onto grains and available for H2CO formation.
Resolved observations of H2CO emission allow us to
con-strain the formation mechanism of H2CO in disks. The H2CO
radial intensity profile in the disk of DG Tau (Fig. 2) shows characteristics in common with the three other disks observed at high angular resolution with ALMA (DM Tau,Loomis et al. 2015; TW Hya,Öberg et al. 2017; and HD 163296,Carney et al. 2017): (i) the depression (or lack) of emission in the inner disk; (ii) the emission peak located outside the CO iceline; (iii) emis-sion beyond the mm dust continuum (with a peak at the edge of the continuum in the case of DG Tau, TW Hya, and HD 163296). In the case of the HD 163296 disk, it was argued that the cen-tral depression of the H2CO emission is caused by absorption
by optically thick dust continuum (Carney et al. 2017), while
Öberg et al. (2017) explain it as a real drop in abundance in TW Hya. For DG Tau, we can exclude problems due to con-tinuum subtraction as the line-free channels of the concon-tinuum subtracted cube do not show negative flux values in the central regions. To evaluate if absorption by the dust continuum affects the observed H2CO intensity, we estimate the optical depth of the
disk at 1.3 mm from the dust opacity and surface density given byIsella et al.(2010). According to their modeling, the disk is optically thick in the innermost region (τ1.3 mm > 1 at <3 au or
<9 au, depending on whether the self-similarity or power-law parametrization for the surface density is adopted). Then the optical depth rapidly drops to ∼0.5 at 20 au and <0.3 for r > 30 au2. On the other hand, observations of13CO 2-1 and C18O
2-1 show a hole in the inner 25 au, which could be due to the thickness of the continuum up to this radius (Güdel et al. 2018). To further check the continuum optical depth we compared its brightness temperature (Tcont, see Fig.2) to the expected dust
temperature Tdust by Podio et al. (2013) (their Fig. 2, where
Tgas = Tdust in the midplane). We found Tcont/Tdust < 0.6,
hence τ < 1, for r > 25 au, which indicates that the con-tinuum should be optically thin beyond this radius, in agree-ment with the observed distribution of13CO 2-1 and C18O 2-1.
Finally, we checked the H2CO 31,2− 21,1line opacity by using
the non-LTE radiative transfer code RADEX (van der Tak et al. 2007). Assuming column densities up to NH2CO ∼ 10
15cm−2
in the inner disk region (10 − 40 au) (Walsh et al. 2014), a line width of ∼6 km s−1, and gas densities and temperatures from our
ProDiMo DG Tau model (nH2 ∼ 10
10−1012cm−3 and T
gas >
30 K at 10−40 au and intermediate disk height), the H2CO line
optical depth reach a maximum value of ∼1. Therefore, we con-clude that the H2CO ring is related to an increase in the H2CO
abundance beyond ∼40 au.
As shown byLoomis et al.(2015), disk models that include only H2CO gas-phase formation produce centrally peaked
col-umn density and emission profiles, and strongly underestimate the observed H2CO emission in the outer disk. H2CO formation
2 At the adopted distance of 121 pc the radius where the disk becomes optically thin is ∼15% smaller with respect to the modeling by
Isella et al.(2010) who adopted d= 140 pc.
on grains followed by desorption in the gas phase has to be taken into account to increase the H2CO column densities and line
emission in the outer disk (by up to two orders of magnitude in the case of DM Tau). The intensity profile of H2CO in the
disk of DG Tau shows a steep increase in the outer disk with an emission peak at 62 au. Interestingly, the thermo-chemical ProDiMo disk model presented in Podio et al. (2013), which matches the long wavelength part of the SED of DG Tau and the Herschel/HIFI water lines, shows that the CO iceline, RCO,
is located at ∼30 au3. The fact that the peak is located outside the CO iceline suggests that the outer H2CO reservoir is mainly
pro-duced by CO hydrogenation on grains. This is in agreement with disk chemistry models and with observations byLoomis et al.
(2015),Öberg et al.(2017) andCarney et al.(2017). Contrary to the disks of DM Tau, TW Hya, and HD 163296, in the case of DG Tau no H2CO emission is detected in the inner disk, where
gas-phase formation dominates. As discussed above, this could be due to absorption by the optically thick dust continuum (up to ∼25 au) and to a lower H2CO abundance (by a factor of ≤10)
and column density in the inner 40 au (see Sect.4). Also taking into account the wider velocity range over which the emission may be spread, this would produce no detectable emission in the present observations (rms ∼1.7 mJy beam−1). Finally, the fact
that the H2CO emission peaks at the edge of the mm dust
con-tinuum and extends beyond it could be due to a steeper decrease in the density of the gas and of the small grains at this loca-tion. As small grains are responsible for the opacity at UV wave-lengths, the UV radiation penetrates deeper in the disk and more efficiently photodesorbs H2CO from grains (Öberg et al. 2015b,
2017). Moreover,Cleeves(2016) suggests that there could be a temperature inversion at the edge of the large dust population, which would enhance thermal desorption. Alternatively, H2CO
could be efficiently formed in the gas phase beyond the mm-dust edge due to more efficient photodissociation of CO favoring the CH3+ O formation route (Carney et al. 2017). An
enhance-ment of molecular emission at the edge of the millimeter dust continuum has been observed in a number of disks (Öberg et al. 2015b,2017;Huang et al. 2016;Carney et al. 2017,2018), but a test of the proposed mechanisms would require higher angu-lar resolution observations of multiple molecules and detailed modeling.
6. Disk chemistry and dust properties
The H2CO 31,2− 21,1ring is asymmetric as the redshifted side
is ∼1.5 times brighter than the blueshifted side (Figs. 1, A.2, andA.3). The same asymmetry is observed in the map of the linearly polarized intensity of the continuum at 0.87 mm by
Bacciotti et al.(2018), where the redshifted disk side is brighter than the blueshifted side in the outer 40−50 au region. The map by Bacciotti et al. (2018) also indicates that the orienta-tion of the linear polarizaorienta-tion vector changes from parallel to the disk minor axis in the inner disk region to azimuthal in the outer 40−50 au. Interestingly, the radius where the change in the orientation of polarization occurs is coincident with the inner radius of the H2CO ring. The intensity and orientation of
the polarization of the dust continuum caused by self-scattering strongly depends on the dust size, opacity, and degree of set-tling (Yang et al. 2017). This suggests that the H2CO chemistry
3 R
CO is derived from a steady-state solution of a chemical network including gas-phase chemistry as well as adsorption and (non)thermal desorption processes (e.g.,Kamp et al. 2010) for a stellar luminosity L∗∼ 1 L at 140 pc. At the adopted distance of 121 pc, L∗decreases by ∼25%, hence the estimated RCOis an upper limit.
is closely connected to the dust properties in the outer disk. For example, a drop in the dust opacity beyond 40 au could be responsible for the change in polarization (Bacciotti et al. 2018). If the opacity of small grains also decreases, this would cause an enhancement of H2CO desorbed from grains, due to increased
UV and X-ray penetration. In the case of DG Tau, the desorption mechanism could be also favored by UV and X-rays produced in the blueshifted jet shocks (Coffey et al. 2007;Güdel et al. 2008) that directly illuminate the disk from above. The chemical and polarization change at 40 au could also be linked to the existence of substructures in the dust distribution. The radial profile of the continuum intensity along the projected disk major axis shows a shoulder of emission at about 40 au (see Fig.B.1), which sup-ports this hypothesis.
Finally, it is interesting to note that the H2CO ring is offset by
∼0.0006 (∼7 au) with respect to the dust continuum peak, i.e., the
redshifted ring side is located farther from the continuum peak than the blueshifted side (Fig.1). This also suggests an asym-metric distribution of the chemically enriched gas in the outer disk region. However, higher angular resolution observations are needed to confirm this scenario.
7. Conclusions
ALMA observations at 0.0015 resolution of the disk of DG Tau show a ring of formaldehyde peaking at ∼62 au, i.e., outside the CO iceline (RCO ∼ 30 au) and close to the edge of the 1.3 mm
dust continuum (Rdust ∼ 66 au). This suggests an enhancement of
the H2CO abundance in the outer disk due to H2CO formation by
CO hydrogenation on the icy grains. Moreover, the desorption of H2CO from the grains could be enhanced at the edge of the mm
dust continuum due to increased UV penetration and/or tempera-ture inversion. The abundance ratio between CH3OH and H2CO
is <1, in agreement with the predictions of disk chemistry mod-els. Interestingly, the inner edge of the H2CO ring is at ∼40 au,
where the polarization of the dust continuum changes orientation and the continuum intensity shows a local enhancement, hinting at a tight link between the H2CO chemistry and the dust
proper-ties in the outer disk and at the possible presence of substructures in the dust distribution.
Acknowledgements. This paper uses ALMA data from project 2016.1.00846.S. ALMA is a partnership of ESO (representing its member states), NSF (USA), and NINS (Japan), together with NRC (Canada), MOST and ASIAA (Taiwan), and KASI (Republic of Korea), in cooperation with the Republic of Chile. The Joint ALMA Observatory is operated by ESO, AUI/NRAO, and NAOJ. This work was partly supported by PRIN-INAF/2016 GENESIS-SKA and by the Ital-ian Ministero dell’Istruzione, Università e Ricerca through the grants Progetti Premiali 2012/iALMA C52I13000140001), 2017/FRONTIERA (CUP-C61I15000000001), and SIR-(RBSI14ZRHR). GG acknowledges the financial support of the Swiss National Science Foundation within the framework of the National Centre for Competence in Research PlanetS.
References
Aikawa, Y., & Herbst, E. 1999,A&A, 351, 233
Bacciotti, F., Girart, J. M., Padovani, M., et al. 2018,ApJ, 865, L12
Balucani, N., Ceccarelli, C., & Taquet, V. 2015,MNRAS, 449, L16
Bianchi, E., Codella, C., Ceccarelli, C., et al. 2017,MNRAS, 467, 3011
Carney, M. T., Hogerheijde, M. R., Loomis, R. A., et al. 2017,A&A, 605, A21
Carney, M. T., Fedele, D., Hogerheijde, M. R., et al. 2018,A&A, 614, A106
Carney, M. T., Hogerheijde, M. R., Guzmán, V. V., et al. 2019,A&A, in press, DOI:10.1051/0004-6361/201834353
Caselli, P., & Ceccarelli, C. 2012,A&ARv, 20, 56
Cleeves, L. I. 2016,ApJ, 816, L21
Coffey, D., Bacciotti, F., Ray, T. P., Eislöffel, J., & Woitas, J. 2007,ApJ, 663, 350
Eislöffel, J., & Mundt, R. 1998,AJ, 115, 1554
Favre, C., Fedele, D., Semenov, D., et al. 2018,ApJ, 862, L2
Fedele, D., Bruderer, S., van Dishoeck, E. F., et al. 2013,A&A, 559, A77
Gaia Collaboration (Prusti, T., et al.) 2016,A&A, 595, A1
Gaia Collaboration (Brown, A. G. A., et al.) 2018,A&A, 616, A1
Garrod, R. T., Widicus Weaver, S. L., & Herbst, E. 2008,ApJ, 682, 283
Güdel, M., Skinner, S. L., Audard, M., Briggs, K. R., & Cabrit, S. 2008,A&A, 478, 797
Güdel, M., Eibensteiner, C., Dionatos, O., et al. 2018,A&A, 620, L1
Guilloteau, S., Di Folco, E., Dutrey, A., et al. 2013,A&A, 549, A92
Guzmán, V. V., Öberg, K. I., Carpenter, J., et al. 2018,ApJ, 864, 170
Huang, J., Öberg, K. I., & Andrews, S. M. 2016,ApJ, 823, L18
Isella, A., Natta, A., Wilner, D., Carpenter, J. M., & Testi, L. 2010,ApJ, 725, 1735
Kamp, I., Tilling, I., Woitke, P., Thi, W.-F., & Hogerheijde, M. 2010,A&A, 510, A18
Kitamura, Y., Kawabe, R., & Saito, M. 1996,ApJ, 457, 277
Loomis, R. A., Cleeves, L. I., Öberg, K. I., Guzman, V. V., & Andrews, S. M. 2015,ApJ, 809, L25
Millar, T. J., Herbst, E., & Charnley, S. B. 1991,ApJ, 369, 147
Müller, H. S. P., Thorwirth, S., Roth, D. A., & Winnewisser, G. 2001,A&A, 370, L49
Öberg, K. I., Qi, C., Fogel, J. K. J., et al. 2010,ApJ, 720, 480
Öberg, K. I., Qi, C., Fogel, J. K. J., et al. 2011,ApJ, 734, 98
Öberg, K. I., Guzmán, V. V., Furuya, K., et al. 2015a,Nature, 520, 198
Öberg, K. I., Furuya, K., Loomis, R., et al. 2015b,ApJ, 810, 112
Öberg, K. I., Guzmán, V. V., Merchantz, C. J., et al. 2017,ApJ, 839, 43
Pacheco-Vázquez, S., Fuente, A., Baruteau, C., et al. 2016,A&A, 589, A60
Podio, L., Kamp, I., Flower, D., et al. 2012,A&A, 545, A44
Podio, L., Kamp, I., Codella, C., et al. 2013,ApJ, 766, L5
Qi, C., Öberg, K. I., & Wilner, D. J. 2013,ApJ, 765, 34
Schuster, K. F., Harris, A. I., Anderson, N., & Russell, A. P. G. 1993,ApJ, 412, L67
Shirley, Y. L. 2015,PASP, 127, 299
Siess, L., Dufour, E., & Forestini, M. 2000,A&A, 358, 593
Testi, L., Bacciotti, F., Sargent, A. I., Ray, T. P., & Eislöffel, J. 2002,A&A, 394, L31
Tielens, A. G. G. M., & Hagen, W. 1982,A&A, 114, 245
van der Marel, N., van Dishoeck, E. F., Bruderer, S., & van Kempen, T. A. 2014,
A&A, 563, A113
van der Tak, F. F. S., Black, J. H., Schöier, F. L., Jansen, D. J., & van Dishoeck, E. F. 2007,A&A, 468, 627
van ’t Hoff, M. L. R., Tobin, J. J., Trapman, L., et al. 2018,ApJ, 864, L23
Walsh, C., Millar, T. J., Nomura, H., et al. 2014,A&A, 563, A33
Walsh, C., Loomis, R. A., Öberg, K. I., et al. 2016,ApJ, 823, L10
Watanabe, N., & Kouchi, A. 2002,ApJ, 571, L173
Willacy, K., & Woods, P. M. 2009,ApJ, 703, 479
Yang, H., Li, Z.-Y., Looney, L. W., Girart, J. M., & Stephens, I. W. 2017,
A&A 623, L6 (2019)
Appendix A: H2CO channel maps, moment 8 map,
and 1D spectrum
The channel maps and the moment 8 map obtained from the H2CO 312−211line cube towards DG Tau are shown in Figs.A.1
andA.2. The moment 8 map strongly supports the ring detection with a S /N ∼ 5.
The 1D spectrum obtained integrating the H2CO line cube
over a circular ring extending from 0.0033 to 0.0075 is shown in Fig.A.3.
Fig. A.1.Channel maps of H2CO 312− 211towards DG Tau. The blue and red contours show the emission at symmetric blue- and redshifted
velocities with respect to systemic (Vsys = +6.24 km s−1), as labeled in the upper right corner (V − Vsysin km s−1). The first contour is at 5σ with steps of 3σ. The black star and contour indicates the peak and the 5σ level of the 1.3 mm continuum. The ellipse in the bottom right corner of the last channel map shows the ALMA synthesized beam.
Fig. A.2.H2CO moment 8 map towards DG Tau. The color scale
indi-cates the line intensity in Jy beam−1. The white ellipse in the bottom left corner indicates the synthesized beam.
3 4 5 6 7 8 9 V LSR [km/s] 0.00 0.02 0.04 0.06 0.08 0.10
Line intensity [Jy]
Fig. A.3. H2CO 31,2 − 21,1 spectrum integrated over a 0.0033−0.0075
ring area. The vertical dashed lines indicate the systemic velocity, Vsys= +6.24 km s−1, and the position of the blue- and redshifted peaks (Vpeak= Vsys± 1.95 km s−1).
Appendix B: Analysis of the continuum radial intensity profile
In Fig.B.1we plot the radial profile of the continuum intensity along the projected disk major axis, in order to achieve the high-est resolution, and avoid the dilution of possible substructures in the dust flux that could result from azimuthal averaging. No strong asymmetries are visible between the northwest and the southeast sides of the disk. A tentative shoulder appears at about 40 au from the star: to characterize its position we computed the second derivative of the radial intensity profile (by calculating the differentials over bins of 5 au, averaged between the NW and SE sides). The radii at which the second derivative becomes negative give us the locations where there is a local increase in the continuum intensity and the profile becomes locally con-vex. This occurs in the central region (<11.5 au), correspond-ing to the peak of the continuum, and in the range between 38 and 44.5 au where the small shoulder is visible in the intensity profile. Hence, the center of the observed continuum intensity enhancement is located at ∼41 au.
Fig. B.1.Radial cut of the continuum intensity map of DG Tau at 1.3 mm:
every point corresponds to a pixel inside a half-beam across the disk major axis (i= 41◦, PA= 128◦). Red dots represent the southeast side of the disk, blue dots the northwest side. The large black dots correspond to 5 au bins computed by averaging the two sides. The black curve is the second derivative of the continuum intensity profile. The solid vertical line indicates the center of the continuum enhancement, at 41 au.
